Mate, I'm about to give you the most valuable advice on this board. Talk it out with her, lay all your cards on the table. But don't let your emotions run wild that you won't be able to control yourself. Tell her how you feel, listen to her, come to a conclusion and a resolution. No matter what happens, there is only an outcome. And both of you have to reach the same decision. By the sounds of her letter she wrote, she's still willing to be there for you and still loves you enough to try to work it out and stay with you. Being a married man myself, you took an oath that you will love and cherish each other and work through the things that trouble your marriage. From your post, all i hear is that you only confronted her lies, made her feel bad but didn't make her realize the consequences. Thus, snowballing this situation every time she was caught. But if you had have voiced your concerns, emotionally, telling her how it hurt you, then it would have hit home for her. If she cares enough to want to stay and "stick it out", then she cares enough to listen and look at how she's hurting you to stop well and truly. If she doesn't stop, even after you have FINALLY opened up emotionally, then you will just have to walk away, one of the hardest things to do...But will be healthier for the both of you.
And to top this all off, since this IS a steroids forum, I have to finish off with something steroid related.

steroids.